Dark Matter Hunt Heats Up
Dark matter is one of the hot topics of the late 20th century (and today). In the Cosmic Times, we first encountered dark matter in the 1965 edition, with the rotation curves of galaxies. However, as we discussed in the notes to "Galaxies Still Misbehaving", earlier evidence for dark matter came from Fritz Zwicky when he observed clusters of galaxies. Zwicky observed that galaxies in the cluster were moving too fast to be gravitationally bound to the visible mass in the cluster. X-ray observations have uncovered further evidence for unseen mass in clusters of galaxies.
The particular discovery discussed is this article is from the ROSAT satellite, an X-ray observing satellite that launched in 1990. ROSAT observed a group of galaxies and found a cloud of hot gas lying between the galaxies. Out in open space a cloud of hot gas would typically dissipate rather quickly. In order to keep a cloud of hot gas together for any length of time, the gas must be gravitationally bound to the galaxies in the group. However, the mass of the galaxies and gas were not enough to bind the hot gas to the group of galaxies. This observation led to the conclusion that there must be more mass in the group of galaxies than could be seen – further evidence for dark matter.
A back-of-the-envelope calculation can be done for the ROSAT data by comparing the kinetic energy of the hot gas to the gravitational energy of the mass of the galaxies plus gas. One of our lesson plans, Dark Matter NASA Conference, goes through the calculations using the real measurements from ROSAT.
Other resources
The following web pages have more detailed information:
